1. Technical Field
The present invention relates to a satellite signal tracking method and a receiver.
2. Related Art
As a positioning system using a satellite signal, there is known a global positioning system (GPS), which is used in a position computing apparatus integrated in a mobile phone, a car navigation apparatus, or the like. In the GPS, a position computation is performed in which position coordinates of the position computing apparatus and a time-watch error are obtained based on information such as positions of a plurality of GPS satellites or pseudo distances from each GPS satellite to the position computing apparatus.
In a case where the positioning system using the satellite signal is solely used, the position computation accuracy is degraded due to various error sources. For this reason, various techniques have been proposed to improve the position computation accuracy. For example, in regard to a flying object having an accelerometer, JP-A-2009-103488 discloses a technique of acquisition/tracking of the GPS satellite signals by reflecting a signal from the accelerometer on an acquisition/tracking circuit of the GPS satellite signal.
The technique disclosed in JP-A-2009-103488 may be directly applied to a mobile object having the acceleration sensor as follows. That is, accelerations detected by the acceleration sensor are integrated to compute a velocity. In addition, a Doppler frequency is computed using a relative velocity of the mobile object with respect to the velocity of the positioning satellite and a viewing direction directed to the positioning satellite from the mobile object. The computed Doppler frequency is reflected on a loop circuit (typically, a phase-locked loop (PLL)) for tracking the satellite signal.
If the Doppler frequency can be appropriately obtained, the acquisition/tracking of the satellite signal can also be appropriately carried out. However, whether or not the Doppler frequency can be appropriately obtained depends on the detection accuracy of the acceleration sensor and the accuracy of the position computation using the positioning signal. If the detection accuracy of the acceleration sensor is low, a velocity necessary to compute the Doppler frequency may not be correctly obtained. If the accuracy of the position computation is low, a viewing direction necessary to compute the Doppler frequency may not be correctly obtained.
In recent years, micro-electro mechanical systems (MEMS) sensors obtained by applying semiconductor micro-machining techniques have been mounted in various consumer appliances as small-sized and inexpensive acceleration sensors. While an MEMS sensor has some advantages such as a small size, a light weight, inexpensive cost, and low power consumption, it has disadvantages in that it is vulnerable to influence from the external environment such as impact, vibration, or temperature change. For this reason, a significant error may be included in the detection result of the acceleration sensor due to the influence from the external environment such as temperature change or vibration. In addition, the accuracy of the position computation may also be significantly degraded due to various reasons including the environment for receiving the satellite signal, such as a multipath environment or a weak electric field environment, a received signal strength, and arrangement of the positioning satellites.